This paper presents a new approach to the tracking of regions in an image sequence. Unlike most other methods, the proposed approach can handle topology changes, i.e., regions may split or merge. This flexibility is naturally embedded into a partial differential equation that solves a minimum description length (MDL) estimation problem. The basic estimation criterion consists of only two terms: the description length of the region shape mismatch and the description length of the region itself, but we show possible extensions to this basic formulation. We minimize the MDL criterion using the level set methodology that inherently accounts for topology changes. We show results for natural data with natural as well as synthetic motion.